1. Field of the Invention
The present invention relates to an elastic wave filter device using elastic waves, such as surface acoustic waves and boundary elastic waves, and, in particular, to a duplexer including a balanced elastic wave filter device and a boundary elastic wave filter device that have a balanced-unbalanced conversion function and that are capable of increasing the output impedance relative to the input impedance.
2. Description of the Related Art
In recent years, surface acoustic wave filters used in an RF stage of a cell phone have been required to have a balanced-unbalanced conversion function, that is, a balun function. Accordingly, longitudinally coupled resonator type surface acoustic wave filters that can easily provide a balun function and operate in a high-frequency range are widely used for a bandpass filter disposed in the RF stage.
In general, a mixer IC having a balanced input or operational input and output (hereinafter this mixer IC is referred to as a “balanced mixer IC”) is connected to the output side of the surface acoustic wave filter having a balanced-unbalanced conversion function.
Typically, the input impedance of a balanced mixer IC is about 150Ω to about 200Ω. In contrast, the input and output impedance of the surface acoustic wave filter used in the RF stage is about 50Ω.
Accordingly, for a surface acoustic wave filter connected to the above-described balanced mixer IC, the output impedance should be about 150Ω to about 200Ω, which is greater than about 50Ω.
Therefore, Japanese Unexamined Patent Application Publication No. 2002-290203 describes a surface acoustic wave filter device having a balanced-unbalanced conversion function and a ratio of the input impedance to the output impedance of about 1:4. FIG. 10 is a plan view schematically illustrating the electrode structure of the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2002-290203.
A surface acoustic wave filter device 501 includes an unbalanced terminal 502, a first balanced terminal 503, and a second balanced terminal 504. First and second 3-interdigital transducer (hereinafter referred to as IDT) longitudinally coupled resonator type elastic wave filter units 505 and 506 are connected in parallel to the unbalanced terminal 502. The longitudinally coupled resonator type elastic wave filter unit 505 includes first to third IDTs 505a to 505c arranged in this order in the surface acoustic wave propagation direction. Reflectors 505d and 505e are disposed on either side of an area in which the first to third IDTs 505a to 505c are disposed in the surface acoustic wave propagation direction. In a similar manner, the longitudinally coupled resonator type elastic wave filter unit 506 includes first to third IDTs 506a to 506c and reflectors 506d and 506e. 
An end of the center IDT 505b is commonly connected to an end of the center IDT 506b and is connected to the unbalanced terminal 502. In addition, an end of the first IDT 505a is commonly connected to an end of the third IDT 505c and is connected to the first balanced terminal 503. An end of the first IDT 506a is commonly connected to an end of the third IDT 506c and is connected to the second balanced terminal 504.
The first to third IDTs 505a to 505c and the first to third IDTs 506a to 506c are arranged such that the phase of a signal flowing from the unbalanced terminal 502 to the first balanced terminal 503 is different from the phase of a signal flowing from the unbalanced terminal 502 to the second balanced terminal 504 by about 180 degrees.
In the surface acoustic wave filter device 501 described in Japanese Unexamined Patent Application Publication No. 2002-290203, the ratio of the impedance on the side of the unbalanced terminal 502 defining an input terminal to the impedance on the side of the first and second balanced terminals 503 and 504 defining an output impedance can be set in the range of about 1:3 to about 1:4. That is, the ratio of the output impedance to the input impedance can be set to about 4.
However, in recent years, there has been a trend for the input impedance of a balanced mixer IC connected downstream of a surface acoustic wave filter having a balanced-unbalanced conversion function to be increased to at least about 200Ω. In some cases, the input impedance is as high as about 600Ω. Accordingly, the above-described impedance ratio needs to be greater than 4, for example, the impedance ratio may be as large as about 12. However, existing surface acoustic wave filter devices, including the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2002-290203, cannot satisfy such a requirement.
In addition, in recent years, boundary elastic wave filters have been developed which have a simplified packaging structure as compared to that of a surface acoustic wave filter. For such boundary elastic wave filters, it is similarly required that boundary elastic wave filters have a balanced-unbalanced conversion function and a high impedance ratio as described above.